This invention relates generally to surface and subsurface corrosion detectors and in particular to a fiber optic magnetic field gradiometer for detecting magnetic gradients generated by electrochemical corrosion reactions.
The flow of metal ions between two metal surfaces occurs during the corrosion process. Since magnetic fields are associated with current flow, a corrosion process has an associated magnetic field. Therefore, corrosion can be located and identified by detecting these magnetic fields.
A variety of techniques have been developed for studying electrochemical corrosion, including actually weighing the material lost from a sample, monitoring the voltage which develops between two corroding electrodes, or even determining the equivalent circuit of a multielectrode cell by measuring its frequency response. However, the above-mentioned techniques are necessarily invasive and/or destructive. It is of interest to detect corrosion currents in a non-invasive non-destructive manner.
Magnetic fields associated with corrosion have been detected and reported by J. G. Bellingham and M. L. A. MacVicar in "SQUID Technology Applied to the Study of Electrochemical Corrosion," (IEEE Transactions on Magnetics, Vol. MAG-23, No. 2, March 1987). There a SQUID gradiometer was used to detect the magnetic fields associated with electrochemical reactions or corrosion. Since a SQUID device must be immersed in liquid helium, its associated dewars and insulation restrict the sensor to be at least 2 centimeters away from the surface. In addition, such bulky dewars make it impractical to actually scan a SQUID sensor over a surface.
Accordingly, it is an object of the present invention to provide a method and apparatus capable of scanning and detecting corrosion at the surface or subsurface of a metal. It is a further object of the invention to provide an apparatus that is small and lightweight. Other objects and advantages of the invention will be more apparent hereinafter in reference to the detailed description of the preferred embodiments and the drawings.